1. Field of the Invention
The present invention relates to a high-density magnetic recording medium and, more particularly, to a magnetic recording medium from which signals are reproduced by a magnetic recording tape system of a so-called linear method for recording signals on a magnetic tape by using a magnetoresistive magnetic head (MR head) or a giant magnetoresistive magnetic head (GMR head) while causing the magnetic head to move in opposite directions with respect to a longitudinal direction of the magnetic tape.
2. Description of Related Art
A magnetic recording medium of so-called thin metal film type has recently been applied to magnetic recording media in a field of video tape recorders and the like in order to achieve far higher image quality and far higher recording density. The magnetic recording medium of a metal thin film type has a construction in which a magnetic layer is formed on a nonmagnetic support in such a manner that a magnetic metal material or a magnetic material such as a Co—Ni-based alloy, Co—Cr-based alloy or Co—CoO-based metal oxide is directly deposited on the nonmagnetic support by vacuum thin film deposition techniques.
Furthermore, in order to improve the electromagnetic conversion characteristics of recording medium of the above-mentioned type to obtain a far higher output therefrom, so-called oblique evaporation has been proposed which obliquely evaporates a magnetic layer material to form a magnetic layer of a magnetic recording medium. Magnetic recording media of the type which have magnetic layers formed by this method have been put to practical use as metal evaporated tapes for high-band 8 mm video tape recorders and digital video tape recorders.
Such a magnetic recording medium of the thin metal film type as described above is superior in coercive force and squareness ratio and its magnetic layer can be formed as an extremely thin layer, so that it has superior electromagnetic conversion characteristics in the short wavelength range and extremely small demagnetization of recording and thickness loss during reproduction. In addition, unlike a magnetic recording medium of a so-called coating type in which a magnetic layer is formed on a nonmagnetic support in such a manner that the nonmagnetic support is coated with a magnetic coating material including-magnetic powder dispersed in a binder, in the magnetic recording medium of the thin metal film, a binder which is a nonmagnetic material is not contained in the magnetic layer, whereby the charging density of a ferromagnetic metal material can be increased and high recording density can be advantageously realized.
The magnetic tape of an oblique evaporation type is fabricated by a method which causes, for example, an elongated nonmagnetic support to run in the longitudinal direction and deposits a magnetic material onto, and forms a magnetic layer on, a main surface of the nonmagnetic support which is in a running state, whereby high productivity and superior magnetic characteristics can be ensured.
On the other hand, as a demand for magnetic recording media, such as magnetic tapes, capable of being used as data streamers has become greater, a demand for magnetic recording media of far higher recording density has increased. Furthermore, instead of related art inductive magnetic heads, magnetoresistive magnetic heads (MR heads) or giant magnetoresistive magnetic heads (GMR heads) have been applied to magnetic heads to be used during reproduction of recorded information. These MR heads and GMR heads are advantageous in terms of improvement of recording density because they can detect even a slight amount of magnetic flux leakage from magnetic layers with high sensitivity.
Each of the MR heads and GMR heads has a detection limit at which its sensitivity to magnetic flux leakage is saturated, so that the MR and GMR heads cannot detect magnetic flux leakage greater than their design limitations. Accordingly, it is necessary to optimize their sensitivity to magnetic flux leakage by decreasing the film thickness of magnetic layers of magnetic recording media.
Two kinds of systems for recording and reproducing magnetic tapes used for data streamers, that is to say, a helical scan system and a linear system, are put to practical use. The helical scan system is a system for performing recording and reproduction by causing a magnetic head disposed on a rotary drum to scan a magnetic tape while rotating at high speed.
The helical scan system not only enables precise recording of tracks but also can theoretically be controlled to accurately scan recorded tracks during reproduction. Accordingly, the helical scan system can achieve high recording density in magnetic tape systems. The helical scan system has found wide practical use such as home video recorders, high-band 8 mm video tape recorders, and digital video tape recorders.
By the way, the linear system is a system for providing tracks arranged on a magnetic tape in a width direction thereof and performing recording in its longitudinal direction. The linear system can readily cause a magnetic tape to run at high speed and, at the same time, can increase the transfer rate of recording and reproduction by arranging a large number of magnetic heads in parallel.
The helical scan system that can achieve high recording density is advantageous in magnetic recording tape systems for camcorders, but the linear system has found wide practical use in data storages capable of using magnetic recording tape systems without the need to greatly limit their cubic volumes. In the market as well, tapes such as DLT (digital linear tape) and LTO (linear tape-open) are mainstream products.
As magnetic tape media for data storages using the linear system, only the magnetic tape of the so-called coating type is used, and the magnetic tape medium of the oblique evaporation type has not been used. This is because, in the helical scan system, relative movement between a magnetic head and a magnetic tape takes place in one predetermined direction, but in the linear system, a magnetic tape and a magnetic head relatively move in opposite directions along the longitudinal direction of the tape.
FIG. 4 is a schematic cross-sectional view of a magnetic tape medium obtained by the oblique evaporation. As shown in FIG. 4, a magnetic layer 102 is formed on a nonmagnetic support 101. The magnetic tape medium formed by the oblique evaporation has a structure in which an axis of easy magnetization along which recorded magnetic bits are arranged is formed not to extend in an in-plane direction of the tape but to rise from a plane of the tape.
Accordingly, during recording and reproduction, if a magnetic head moves in a forward direction (in a direction indicated by an arrow A in FIG. 4) in sliding contact with an orthorhombic structure of the obliquely evaporated thin film, good recording and reproduction characteristics can be achieved. However, if the magnetic head slides in a reverse direction (in a direction indicated by an arrow B in FIG. 4) with respect to the orthorhombic structure of the obliquely evaporated thin film, characteristics such as optimum recording current, phase characteristic, CN ratio and output characteristic are inferior compared to a case where the magnetic head slides in the forward direction, so that there is a disadvantage that satisfactory recording and reproduction characteristics cannot be obtained.
Accordingly, magnetic tape media using oblique evaporation have rarely been used in the linear system which performs recording and reproduction in the opposite directions. However, a method of constructing a magnetic layer of an obliquely evaporated tape by forming two layers of obliquely evaporated film having mutually different growth directions has been proposed as a method for solving a problem that recording and reproduction characteristics differ between a case where a magnetic head slides in the forward direction with respect to the orthorhombic structure of an obliquely evaporated film and a case where the magnetic head slides in the reverse direction with respect to the orthorhombic structure of the obliquely evaporated film (refer to Patent Document 1). FIG. 5 is a schematic cross-sectional view of a magnetic tape medium described in Patent Document 1.
As shown in FIG. 5, a magnetic layer 102 is formed on a nonmagnetic support 101, and the magnetic layer 102 has a structure in which a lower ferromagnetic metal thin film 102a and an upper ferromagnetic metal thin film 102b are stacked. The orthorhombic structures of the respective lower and upper ferromagnetic metal thin films 102a and 102b are grown in directions mutually opposite along the longitudinal direction of the nonmagnetic support 101. The orthorhombic structures of the respective lower and upper ferromagnetic metal thin films 102a and 102b are optimized to decrease difference in recording and reproduction characteristics between the forward and reverse directions.
Patent Document 2 also discloses a magnetic recording medium in which two layers of obliquely evaporated film are stacked which respectively have orthorhombic structures whose growth directions are different from each other. According to the magnetic recording medium described in Patent Document 2, the ratio of the minimum value to the maximum value of a coercive force obtained when an applied field angle is varied from 0° to 180° is set to 0.65 or more, thereby improving the recording and reproduction characteristics relative to both directions.
Patent Document 3 discloses a magnetic recording method which enables recording and reproduction based on a linear system by setting a length of a magnetic layer made of a single-layered cobalt-based obliquely evaporated film to not greater than ½ of a recording-head gap length. According to this method, a cobalt-based obliquely evaporated film having a thickness of 40 nm or less and a coercive force of 1,800 Oe is suitably employed.
Patent Document 1 is Japanese Patent Application Laid-Open No. 4-353622.
Patent Document 2 is Japanese Patent Application Laid-Open No. 9-73621.
Patent Document 3 is Japanese Patent Application Laid-Open No. 2000-339605.
Non-Patent Document 1 is Magneto-Resistive Heads and Spin Valve Heads: Fundamentals & Applications, Second Edition, translated by Kazuhiko Hayashi, published by Maruzen CO., LTD. in 2002.